Arrangement and a Method For Heating Metal Objects

ABSTRACT

The invention relates to a an arrangement and a method for heating metal objects such as metal bars by convection. The metal objects are passed on a conveyor through a series of chambers where hot air is blown through the conveyor. The temperature is controlled by a sensor connected to a control devise such as a thyristor.

FIELD OF THE INVENTION

The present invention generally relates to the field of heating metal objects, in particular such objects as elongate metal bars.

BACKGROUND OF THE INVENTION

It is a known practice to heat a metal object before performing a working operation on the object. In for example U.S. Pat. No. 4,343,209, a method is disclosed in which bars are heated by a heating coil at localized zones along the length of the bars. The bars are incrementally advanced one piece length at specified intervals through the heating coil to a shear. After each incremental advance of a bar, a piece is sheared from the end of the bar at the then presented heated zone. It is stated that the disclosed method makes it possible to shear without substantial cracking or shattering.

It is an objective of the present invention to provide an improved arrangement and an improved method for heating metal objects in order to facilitate cutting of the metal objects. Such an improved arrangement and improved method may advantageously be used for the purpose of heating metal objects that will subsequently be cut into smaller pieces.

DISCLOSURE OF THE INVENTION

The invention relates to an arrangement for heating metal objects. The inventive arrangement comprises a shell that envelops an inner space. The shell has an entry opening and an exit opening. Inside the shell inner walls divide the inner space into a rear chamber, at least one intermediate chamber and a forward chamber. A permeable conveyor extends from the entry opening of the shell to the exit opening of the shell through the rear, intermediate and forward chamber. In this way, objects loaded on the conveyor can be transported through the chambers. In the at least one intermediate chamber, a fan is arranged to direct a flow of gas towards the conveyor and objects placed on the conveyor. A heating element is also arranged in the intermediate chamber. The heating element may heat a gas flowing from the fan before the gas reaches the conveyor. The chambers are connected to each other in such a way that at least one outlet of the at least one intermediate chamber communicates with the rear chamber, at least one outlet of the intermediate chamber communicates with the forward chamber and at least one inlet of the intermediate chamber communicates with outlets of the rear and forward chambers. In this way, operation of the fan will generate a rear circulation of gas passing twice through the conveyor and a forward circulation of gas also passing twice through the conveyor.

In one embodiment, a temperature sensor is located inside the shell and connected to a control device that controls the temperature of the heating element as a function of a nominal value and a valued indicated by the temperature sensor. The temperature sensor may be placed in the forward chamber between the conveyor and outlet of the forward chamber.

In some embodiments, the arrangement comprises a plurality of fans and a plurality of heating elements arranged in rows extending perpendicularly to the direction of movement of the conveyor. The heating arrangement may be followed by a cutting device that cuts metal objects that have been heated by the heating arrangement.

The invention also relates to a method for heating metal objects. The method comprises passing the metal objects on a permeable conveyor through an inner space enclosed by a shell and divided by inner walls into a rear chamber, at least one intermediate chamber and a forward chamber. As the conveyor passes through the at least one intermediate chamber, air is heated and the heated air is blown through the conveyor. A first part of the heated air that has passed through the conveyor in the at least one intermediate chamber is diverted into a rear circulation of air. The rear circulation of air is passed through the conveyor in the rear chamber and back to the at least one intermediate chamber. A second part of the heated air that has passed through the at least one intermediate chamber is diverted into a forward circulation of air. The forward circulation of air passes through the conveyor in the forward chamber and back to the at least one intermediate chamber.

In an embodiment of the invention, the temperature of the air is measured inside the shell and the heating of air is controlled as a function of a nominal value and the measured value. A suitable nominal value for the temperature may be 70° C. If the temperature is measured, this may be done in the forward chamber at such a location that the temperature of the air is measured after it has passed through the conveyor in the forward chamber.

In an advantageous embodiment, the air is heated by a heater located inside the at least one intermediate chamber and the temperature of the heater is kept in the range of 200° C.-300° C.

The metal objects are elongate metal bars. The heating operation may advantageously be followed by a subsequent operation such as cutting of metal bars. The heating operation itself can then be regarded as a part of a method for heating and cutting metal bars.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross sectional view of a heating arrangement according to the present invention.

FIG. 2 is a cross-sectional view along the line II-II in FIG. 1.

FIG. 3 is a schematic representation of a cutting device.

FIG. 4 illustrates the operation of the cutting device shown in FIG. 3.

FIG. 5 is a schematic cross sectional view of a heating arrangement according to an alternative embodiment.

DETAILED DESCRIPTION OF THE INVENTION

With reference to FIG. 1, the invention relates to an arrangement 1 for heating metal objects 19, in particular as a preliminary operation that precedes a subsequent cutting operation. In the embodiment illustrated in FIG. 1, metal objects such as elongate bars 19 are placed in a storage 20. The metal objects 19 can be loaded on a conveyor 17 and transported through the heating arrangement 1. In order to dispense objects such as elongate bars 19 from the storage 20, the storage 20 may be provided with one or several flexible elements 37 on which the metal objects rest in the storage 20. If the flexible element 37 is shortened, the bars 19 are lifted and gravity will cause the bars 19 to fall on the conveyor 17. The flexible element 37 may be, for example, one or several belts 37 that can be made shorter by winding an end of the belt 37 on a reel (not shown). In the embodiment illustrated in FIG. 1, the conveyor 17 is provided with teeth 18 that makes transfer to the conveyor more reliable and also keeps the bars 19 spaced from each other on the conveyor 17.

The heating arrangement 1 comprises a shell 2 that envelops an inner space 3. The shell 2 has an entry opening 4 and an exit opening 5. Inside the shell 2, inner walls 6, 7 divide the inner space 3 into a rear chamber 8, at least one intermediate chamber 9 and a forward chamber 10. The conveyor 17 is permeable to air and extends from the entry opening 4 of the shell 2 to the exit opening 5 of the shell 2 through the rear 8 intermediate 9 and forward chamber 10. In this way, objects 19 loaded on the conveyor 17 can be transported through the chambers 8, 9, 10 from the entry opening 4 to the exit opening 5. In the embodiment of FIG. 1, the conveyor extends beyond the shell 2 on both sides of the shell 2. The conveyor 17 may be followed by a cutting device 31 that is symbolically indicated in FIG. 1. After a metal object such as an elongate bar 19 has been heated in the heating arrangement 1, it is somewhat softer and can more easily be cut into smaller pieces by the cutting device 31.

In the at least one intermediate chamber 9, a fan 21 is arranged to direct a flow of air (or possibly another gas) towards the conveyor 17 and objects 19 placed on the conveyor 17. A heating element 22 in the intermediate chamber 9 is arranged to heat air flowing from the fan 21 before the air reaches the conveyor 17. In the embodiment illustrated in FIG. 1, the fan 21 is driven by a motor M which may be an electric motor M. The electric motor M is preferably located inside the intermediate chamber 9 together with the fan 21.

The chambers 8, 9, 10 are connected to each other in such a way that at least one outlet 14 of the at least one intermediate chamber 9 communicates with the rear chamber 8, 10. At least one outlet 14 of the intermediate chamber 9 communicates with the forward chamber 10 and at least one inlet 13 of the intermediate chamber 9 communicates with outlets 12, 16 of the rear and forward chambers 8, 10. In this way, operation of the fan 21 will generate a rear circulation of air passing twice through the conveyor 17 and a forward circulation of air also passing twice through the conveyor 17. As indicated in FIG. 1, a separating wall 23 may be located at the outlet 14 of the intermediate chamber 9. The separating wall 23 serves to separate the airflow from the intermediate chamber 9 into a forward circulation of air and a rear circulation of air.

In one embodiment, a temperature sensor 25 is located inside the shell 2 and connected to a control device 26 that controls the temperature of the heating element 22 as a function of a nominal value and a valued indicated by the temperature sensor 25. The temperature sensor 25 is preferably placed in the forward chamber 10 between the conveyor 17 and outlet 16 of the forward chamber 10 but it could also be located on other places, for example in the intermediate chamber. The heating element 22 may be, for example, an electric coil.

As indicated in FIG. 2, embodiments of the inventive arrangement may comprise a plurality of fans 21 and a plurality of heating elements 22 arranged in rows extending perpendicularly to the direction of movement of the conveyor 17. In a realistic embodiment of the invention, the arrangement could comprise 12 fans 21 and 12 heating elements 22. Each heating element 22 may be dimensioned for about 3500 W in a realistic embodiment. With 12 heating elements, the entire heating arrangement would then be capable of 42 kW.

The operation of the inventive arrangement is as follows. Metal objects 19 are passed on the permeable conveyor 17 through the inner space 3 enclosed by the shell 2 and through the chambers 8, 9, 10 of the inner space. In FIG. 1, the metal objects 19 move through the inner space 3 from left to right in the direction of arrow A. Air is blown by the fan 21 towards the permeable conveyor 17 and through the conveyor 17 as the conveyor 17 passes through the intermediate chamber 10. Before the air reaches the conveyor 17, the air is heated by a heating element 22 which may be an electrical heating element 22. When the heated air passes through the conveyor 17, the metal objects 19 are heated by the air. The permeable conveyor 17 and the objects 19 loaded on the conveyor act as a throttle that forces the air to increase its velocity as it passes through the conveyor 17. The increased velocity of the air improves the heat transfer to the metal objects 19. In a realistic embodiment, the speed of the air may be about 8-10 m/s when the hot air passes through the conveyor 17. Preferably, there is no other throttle than the one represented by the conveyor 17. In other parts of the system, air flow can be relatively slow which reduces losses due to friction. Immediately downstream of the fan 21, the velocity of the air may be about 4 m/s in a realistic embodiment of the invention.

When the hot air has passed the conveyor 17 and transferred a part of its heat energy to the metal objects 19, a first part of the heated air is diverted into a rear circulation of air RA that is passed to the rear chamber 8, through the conveyor 17 in the rear chamber 8 and back to the at least one intermediate chamber 9. A second part of the heated air that has passed through the conveyor 17 in the intermediate chamber 9 is diverted into a forward circulation of air FA that is passed to the forward chamber 10 and through the conveyor 17 as the conveyor 17 passes through the forward chamber 10. The air in the forward circulation of air FA is then sent back to the intermediate chamber 9.

It is preferable that the motor M of the fan 21 be located inside the shell 2 of the arrangement. Locating the motor M outside the shell 2 would result in a more complicated design. If the motor M is an electric motor, is must realistically be expected that the motor will fail or collapse if the ambient temperature becomes to high. For currently available electric motors that the inventors are aware of, ambient temperature should preferably not exceed 70° C. In a preferred embodiment of the invention, the temperature of the air is measured inside the shell 2 and the heating of air is controlled as a function of a nominal value and the measured value. In the embodiment illustrated in FIG. 1, a temperature sensor 25 is located in the forward chamber 10 adjacent the conveyor 17 such that the temperature of the air is measured after it has passed through the conveyor 17 for the last time before it reaches the fan 21 once more. While the temperature sensor 25 could be located elsewhere, this particular location is considered suitable because the temperature of the air will not change much from this point until it reaches the fan 21 again. Other suitable locations for the sensor 25 could be further downstream in the forward circulation FA, i.e. closer to the fan 21. The position of the sensor 25 could thus be anywhere in the forward chamber 10 between the conveyor 17 and the outlet 16 of the forward chamber. The sensor 25 could also be located in the intermediate chamber upstream of the fan 21 and its motor M. As indicated in FIG. 1, the temperature sensor 25 may be connected by a cable 28 to a control device 26 such as for example a thyristor. In the embodiment shown in FIG. 1, the thyristor 26 may be connected by a cable 30 to a further control device 27, for example a computer 27. The thyristor 26 may also be connected by a cable 29 to the heating element 22. The computer 27 gives a nominal value for the temperature to the thyristor 27 while the actual value for the temperature is given by the sensor 25 through the cable 28. The thyristor can then control the output of the heating element(s) 22 to increase or decrease the output. In order to protect the motor M of the fan 21, the temperature measured by the temperature sensor 25 should preferably not be allowed to exceed 70° C. This means that the nominal value given by the computer 27 is 70° C. Of course, to the extent that the motor M is able to function reliably at temperatures over 70° C., the temperature measured by the temperature sensor 25 could be allowed to be correspondingly higher. In the embodiment shown in FIG. 1, cables or wires 28, 29, 30 connect the thyristor 26 to the heating element, the temperature sensor 25 and the computer 27. However, the thyristor 26 could also have a wireless communication with these elements.

If electric coils are used as heating elements, the temperature of the coil or coils 22 could be kept in the range of 22 is kept in the range of 200° C.-300° C. such that the air that reaches the conveyor in the intermediate chamber 9 has a temperature which is also in the range of 200° C.-300° C. The life expectancy of electric coils is highly dependent on the temperature to which the coils are heated. For such coils that are currently commercially available, the life expectancy of the coils is almost unlimited if the temperature does not exceed 300° C. Of course, the temperature of the air will be much lower after the air has passed twice through the conveyor 17 and a part of the heat in the air has been transferred to the metal objects 19. The metal objects to be heated could normally be heated to temperatures well over 300° C. without harmful effects to the metal objects themselves. The temperatures employed are instead limited by the motor M or the desired life expectancy of the coils 22.

As previously mentioned, the heating arrangement 1 may be followed by a cutting device 31. With reference to FIG. 3, a possible design of a cutting device will now be explained. The cutting device may comprise a fixed tool 32 and a movable tool 33. A channel 34 extends through the fixed tool 32 and the movable tool 33. A punch 35 is arranged to strike the movable tool 33. A dampening element 36 may be included to receive the impact from a cutting stroke and return the movable tool 33 to its original position. To cut a metal bar 19, the bar is inserted into the channel 34 such that it extends through the fixed tool 32 and into the movable tool 33. The punch 35 strikes the movable tool 33 as indicated in FIG. 4 and the bar 19 is cut. Of course, a suitable cutting device 31 could take many other forms. If the bars have been heated before the cutting operation, the cutting will be easier and the surface at the actual cut can be smoother and more even.

In one realistic embodiment, the heating arrangement and method could be used to heat stainless steel bars having a length of about 5.5 m and a diameter of 16-25 mm. With currently available cutting devices, such bars can subsequently be cut into 180 pieces in a period of about 50 seconds. This means that new bars must be continuously and rapidly supplied.

The inventors have found that the use of heated air entails an advantage compared to the use of heating by induction. When objects such as elongate metal bars 19 are heated, it may be difficult to achieve an even heating of the bars when inductive heating is used. For example, when elongate bars of stainless nickel steel are heated by induction, the surface of the bars may become heated to a temperature that is harmful while inner parts of the bar are still to cold to allow the bar to be cut as easily as one would desire. Moreover, the inventors have found that objects with a bright surface are more difficult to heat by induction. Objects with a bright surface could include objects of stainless steel. Stainless steel bars have a poor thermal conductivity which makes it more difficult to heat such objects. The inventors have found that heating metal objects such as stainless steel bars by convection is more efficient than heating the metal objects by induction. A practical way of heating by convection is to pass the bars through a flow of heated air. As previously indicated, one realistic embodiment of the invention may comprise 12 fans and 12 heating coils where each heating coil has a capacity of 3.5 kW such that the entire heating arrangement is capable of 42 kW. To achieve a comparable heating effect on stainless steel bars by induction, the required effect would be about 100 kW. Moreover, the induction heating could cause harm to the surface of the stainless steel bars.

One practical problem associated with the heating process is that the metal objects heated by the hot air may have very different starting temperatures. Bars to be cut may often be stored outdoors and may be taken from an outdoor storage directly to cutting. Depending on the season and the climate, the temperature of the bars may well vary from less than −30° C. to above +30° C. This means that the requirements on the heating operation can vary a lot. Moreover, the size of the objects, for example the diameter of stainless steel bars, also influence the amount of heating that is needed. In order to control the temperature of the bars to be cut, it is desirable that the amount of heating can be controlled. The use of the temperature sensor and control equipment such as the thyristor makes it possible to control the temperature to which the bars are heated. This ensures that, when the bars reach the cutting device, the bars will always have the same temperature. In addition, the motor M of the fan can be protected from heat and it is easier to ensure a high life expectancy for the coils 22.

The division of the heating device into three separate chambers 8, 9, 10 is a very advantageous design for the following reason. Immediately downstream of the fan 21, there will be an overpressure during operation of the fan. However, the throttle formed by the conveyor 17 in combination with the suction effect that the fan 21 generates in the rear chamber 8 and the forward chamber 10 will have the effect that there will be an underpressure in the rear and forward chambers. This counteracts leakage of heated of heated air to the environment. Consequently, this has the advantage that less energy will be required for operation of the process.

For heating stainless steel bars having a diameter of 16-25 mm, it may be suitable to keep the bars spaced from each other on the conveyor by about 50 mm. The spacing is determined by the distance between the teeth 18 of the conveyor 17. In realistic embodiments of the invention, the distance from the entry opening 4 to the exit opening 5 may be about 700-1000 mm which allows about 14-20 bars to be inside the heating arrangement 1 at the same time.

It should be understood that the temperature control system described above could be used also for a heating arrangement which is not divided into three separate chambers. It should also be understood that the idea of using three separate chambers can be used independently of whether temperature is measured and controlled or not. However, the temperature control is preferably combined with the principle of using three separate chambers.

Possibly, there could be two or more intermediate chambers, each having its own set of fans 21 and heating elements 22.

An alternative embodiment will now be explained with reference to FIG. 5. In FIG. 5, the rear chamber 8 according to the embodiment of FIG. 1 has been completely eliminated and the entry opening 4 leads directly into the chamber 9 where the fan(s) 21 and the heating element(s) 22 is/are located. In this embodiment, there is no intermediate chamber but only an entry chamber 9 and an exit chamber 10.

An alternative way of describing this embodiment would be to say that there is only a first chamber 9 and a second chamber 10, the fan(s) 21 and heating element(s) 22 being located in the first chamber 9.

Except for the elimination of the rear chamber 8, the embodiment of FIG. 5 functions in substantially the same way as the embodiment of FIG. 1. Since the heater(s) 22 and the fan(s) 21 is/are located in the first chamber 9 that borders the outside of the shell 2, it can be expected that losses of heat through the entry opening 4 will be larger than in the embodiment according to FIG. 1. Consequently, the embodiment of FIG. 5 is believed by the inventors to be less advantageous than the embodiment of FIG. 1. However, leakage of hot air to the ambient atmosphere will still be reduced to some extent since there will still be an underpressure in the second chamber 10 which counteracts leakage of hot air through the exit opening 5.

Hence, the invention can be described in a more general way as relating to an arrangement and a method where air is heated in one chamber 9 and blown by a fan or fans 21 through a permeable conveyor 17 and at least a part of the air flow is diverted to at least one additional chamber 8, 10 and passed back in a loop to the chamber 9 in which the air has first been heated. In principle, this could even include embodiments with two chambers where heating takes place in the second chamber 10, even if such an embodiment is believed by the inventors to being less advantageous compared to the embodiments of FIG. 1 and FIG. 5.

While the invention has been described above in terms of an arrangement and a method, it should be understood that these categories reflect different aspects of the invention. Consequently, the inventive method may include any method step that would be the natural consequence of operating the inventive arrangement, regardless of whether such steps have been explicitly mentioned or not. 

1) An arrangement for heating metal objects, the arrangement comprising: a) a shell that envelops an inner space, the shell having an entry opening and an exit opening; b) inside the shell, inner walls dividing the inner space into a rear chamber, at least one intermediate chamber and a forward chamber; c) a permeable conveyor extending from the entry opening of the shell to the exit opening of the shell through the rear, intermediate and forward chamber such that objects loaded on the conveyor can be transported through the chambers; d) in the at least one intermediate chamber, a fan arranged to direct a gas flow towards the conveyor and objects placed on the conveyor; e) in the at least one intermediate chamber, a heating element arranged to heat a gas flowing from the fan before the gas reaches the conveyor; f) and wherein the chambers are connected to each other in such a way that at least one outlet of the at least one intermediate chamber communicates with the rear chamber, at least one outlet of the intermediate chamber communicates with the forward chamber and at least one inlet of the intermediate chamber communicates with outlets of the rear and forward chambers such that operation of the fan will generate a rear circulation of gas passing twice through the conveyor and a forward circulation of gas also passing twice through the conveyor. 2) An arrangement according to claim 1, wherein a temperature sensor is located inside the shell and connected to a control device that controls the temperature of the heating element as a function of a nominal value and a valued indicated by the temperature sensor. 3) An arrangement according to claim 2, wherein the temperature sensor is placed in the forward chamber between the conveyor and outlet of the forward chamber. 4) An arrangement according to claim 1, wherein the arrangement comprises a plurality of fans and a plurality of heating elements arranged in rows extending perpendicularly to the direction of movement of the conveyor. 5) A method for heating metal objects comprising the steps of: a) passing the metal objects on a permeable conveyor through an inner space enclosed by a shell and divided by inner walls into a rear chamber, at least one intermediate chamber and a forward chamber; b) heating air and blowing the heated air through the conveyor as the conveyor passes through the at least one intermediate chamber; c) diverting a first part of the heated air that has passed through the conveyor in the at least one intermediate chamber into a rear circulation of air that is passed through the conveyor in the rear chamber and back to the at least one intermediate chamber; and d) diverting a second part of the heated air that has passed through the at least one intermediate chamber into a forward circulation of air that is passed through the conveyor in the forward chamber and back to the at least one intermediate chamber. 6) A method according to claim 5, wherein the temperature of the air is measured inside the shell and the heating of air is controlled as a function of a nominal value and the measured value. 7) A method according to claim 6, wherein the temperature is measured in the forward chamber at such a location that the temperature of the air is measured after it has passed through the conveyor in the forward chamber. 8) A method according to claim 5 wherein the air is heated by a heater located inside the at least one intermediate chamber and the temperature of the heater is kept in the range of 200° C.-300° C. 9) A method according to claim 6, wherein the nominal value is 70° C. 10) A method according to claim 5, wherein the metal objects are elongate metal bars. 11) An arrangement for heating metal objects, the arrangement comprising: a) a shell that envelops an inner space, the shell having an entry opening and an exit opening; b) inside the she, at least one inner wall dividing the inner space into separate chambers; c) a permeable conveyor extending from the entry opening of the shell to the exit opening of the shell through separate chambers such that objects loaded on the conveyor can be transported through the chambers; d) in at least one of the chambers, at least one fan arranged to direct a gas flow towards the conveyor and objects placed on the conveyor; e) at least one heating element arranged to heat a gas flowing from the at least one fan before the gas reaches the conveyor; f) and wherein the chambers are connected to each other in such a way air which has been heated in one chamber and blown by the at least one fan through the conveyor is diverted to at least one additional chamber and passed back in a loop to the chamber in which the air has first been heated, the additional chamber being a chamber with an entry opening or an exit opening for the conveyor. 